Despite having been introduced ten years ago, little is known of the impact of electronic nicotine delivery systems (ENDS or e-cigarettes or e-cigs) on human health. ENDS usage is rapidly rising among adolescents and young adults and this is partially due to a perception that e-cigs are safer than cigarettes and other nicotine replacement therapies. However, these products vary widely in nicotine content, propylene glycol, glycerol, additives and flavorings; all of which are poorly studied. Therefore, there is an urgent need for biological investigations that will improve our understanding of how ENDS impact human health. The initial point of contact of ENDS, and the first-affected system in humans is the oral cavity. It is known that nicotine impacts oral health, acting as a cytotoxic agent as well as increasing virulence of certain bacteria. Oral health is dependent upon an intricate balance between the microbial ecosystem and host immunity. Based on our preliminary studies that ENDS alter the compositional structure and the functional potential of the oral ecosystem, we hypothesize that e-cigs will impact this microbial-mucosal homeostasis. We propose to test this hypothesis by combining novel clinical study designs with integrated `-omics' approaches and computational bioinformatics. This approach will bridge the gap between purely clinical outcome-based studies, in vitro investigations and in vivo investigations using animal models. First, the biological effects of different formulations of ENDS on an in vitro model of the oral host-microbial interactome will be examined through dual transcriptome-metatranscriptome sequencing. Second, we will observe the effects of ENDS on host-bacterial interactions over time by combining a multi-arm, longitudinal clinical study with a network analysis algorithm. The proposed studies will bring us closer to understanding the role of ENDS in disrupting oral ecological homeostasis, provide biologically-validated timelines for assessing risk associated with ENDS use, and develop indicators or predictors of exposure.